This invention relates generally to an X-ray image intensifier tube and more particularly to a method of using a fiber optic plate to increase the contrast ratio of an X-ray image intensifier.
An image intensifier generally comprises a photocathode on which an input image is formed, an amplifying mechanism for enhancing the density of the beam of electrons emitted from the photocathode and an output assembly including a phosphor screen on which the electron beam strikes, converting its energy into visible light energy.
The contrast ratio of such an image intensifier tube is the ratio of brightness intensity observed at a specified place on the output phosphor screen with and without a lead disk of specified diameter placed at the input to obstruct incoming photons such as X-ray photons. The measurement may be made by obstructing the input surface only partially and comparing brightness in the unobstructed field and the image center of the lead disk.
One of the methods of improving the contrast ratio of an imaging device has been to use a fiber optic output faceplate together with a photocathode-amplifier-phosphor combination. An example of such an X-ray imaging device was disclosed in U.S. Pat. No. 4,142,101 issued Feb. 27, 1979 to L. I. Yin, and FIG. 1 schematically illustrates its output assembly 10. A flow of electrons from a photocathode means at the input end of the device strikes a phosphor layer 12 and the electron energy is converted back to light which is piped out for viewing by a fiber optic plate 13 with individual fibers aligned parallel to one another in the same direction as the electron flow. Contrast ratio is thereby enhanced because each fiber responds to the phosphor illumination on the electron-bombardment side and transmits it to the viewing side without substantial illumination of the adjacent fiber, or there is only a minimum of light "cross-talk" between fibers.
These elements 12 and 13 are contained within a housing 15 made of a material suitable for electrical insulation and vacuum isolation such as ceramic or glass. The phosphor layer 12 is connected to a pin 17 which protrudes from the housing 15 to provide a power supply connection. During a typical operation, the phosphor layer is at several kilovolts. The fiber optic plate 13 fused to the housing 15 along the edge of the output window so as to form a complete vacuum seal enclosing an evacuated area 19. Thus, the fiber optic plate 13 must also be able to function as an insulator because the housing 15 must be kept at ground potential during operation. The minimum thickness required of the fiber optic plate 13 to satisfy these conditions is about 5 mm. With thickness of this magnitude, however, both the probability of blemishes and light loss in the fiber optic increase and vacuum leaks still tend to occur.